Jabber is an open alternative to closed instant messaging (IM) and presence services. At its core is the Extensible Messaging and Presence Protocol (XMPP), which defines how to stream XML content and is being used to build not only a large open IM network but also a wide range of XML applications. This article provides an overview of Jabber/XMPP protocols and technologies, as well as an introduction to XMPP-based applications.